1. Field of the Invention
The present invention relates to a controller and a controlling method of a laser machining device.
2. Description of the Related Art
Prior to laser machining by a laser machining device, an approach operation is carried out in order to position a nozzle from which laser is radiated at a target position away from a workpiece by a predetermined distance. A common laser machining device is designed such that the approach operation ends when positioning of the nozzle is completed, and thereafter a subsequent process is carried out.
FIGS. 10A to 10D show an approach operation of a nozzle of a laser machining device. FIG. 10A is an ideal example in which a flat plate-like workpiece W is disposed at a predetermined position. In order to carry out a subsequent process, a nozzle NZ is positioned at a target position T distant from the workpiece W by a target gap amount. In this case, since the position of the workpiece W and the target gap amount are already determined, the precise positioning of the nozzle can be carried out within a short period of time, by simply moving the nozzle NZ in accordance with a position command.
However, in an actual application, the state of the workpiece W is often unknown. For example, FIG. 10B shows the workpiece W warping upward. In such a case, it is necessary to detect an unknown gap amount between the workpiece W and the nozzle NZ, and based on the detected gap amount, the nozzle NZ is moved to a target position T.
However, in a commonly-used optical gap sensor or capacitance gap sensor, accuracy in detecting the gap amount tends to decrease, when the nozzle NZ is at a relatively distant position from the workpiece W. In addition, a delay may occur due to detection of a gap amount and a position control of the nozzle NZ based on the gap amount. As a result, deceleration of the nozzle NZ may be delayed, possibly resulting in undershoot of the nozzle NZ. Therefore, for example, in the case where the warp of the workpiece W is substantial (see FIG. 10C) or an object nearby (for example, cut pieces produced as a result of a cutting process) protrudes upward from the workpiece W, there is a risk of the nozzle NZ coming into collision with the workpiece W, or the like.
If the approach operation is carried out at a low speed, the collision of the nozzle NZ can be prevented even in the case of FIGS. 10C and 10D. However, if the approach operation is carried out consistently at a low speed, the efficiency substantially decreases. In the case where the nozzle NZ merely comes in contact with the workpiece W, there is a possibility that defect of the workpiece may be avoided, and a machining process can continue. On the other hand, if the nozzle NZ comes in contact with the workpiece W during a subsequent process including laser radiation, there is a risk that a part of the workpiece W may be melted off onto other part of the workpiece W, and defect of the workpiece W may spread over a wide range. If this is the case, it may take extensive time to restore the system, for example, it may be necessary to carry out an additional process for removing the workpiece which can be no longer used. Accordingly, it is desirable to complete an approach process of the nozzle in the shortest possible time, while avoiding a risk of producing defect of the workpiece.
JP-A-9-308979 discloses a laser machining device designed to shorten the necessary time for an approach operation from a retracted position to a reference position of profile control. This laser machining device designed to control when to start deceleration, based on a stopping distance determined by a predetermined approach speed and deceleration, and on a distance from the reference position obtained by a gap sensor.
JP-A-2004-001067 discloses a laser machining device designed to shorten the necessary time for moving from a terminating point in a first process to a starting point in a following second step. In this laser machining device, a machining nozzle is moved to a position distant from the workpiece by a predetermined gap amount according to a position command, and thereafter, controlled according to a gap control using feedback of the gap amount detected by a sensor.
JP-A-2000-052076 discloses a laser machining device designed to stably increase an approach speed of a machining head. This laser machining device is controlled such that the machining head is moved at a higher speed until it becomes possible to detect a gap amount between the machining head and the workpiece by a gap sensor, and thereafter, the approach operation is switched to a lower speed.
JP-A-2011-210245, JP-A-10-260734, JP-A-9-190968, JP-A-2002-318361, and JP-A-2012-084631 disclose various common techniques for drive control, although they are not intended to control a laser machining device.
However, even if the controlling methods disclosed in JP-A-9-308979, JP-A-2004-001067, and JP-A-2000-052076 are adopted, it is still necessary to position a nozzle based on a target gap amount. Accordingly, there is a need to shorten the approach time even further.